The present invention relates to a discharge excitation gas laser apparatus, and more particularly, to one having a preionization means for preionizing a laser gas.
In order to effectively excite through electronic discharge a gaseous medium at a pressure equal to or higher than the atmospheric pressure in a gas layer apparatus, it is necessary to produce a spatially uniform glow discharge. To this end, it is general practice to perform preionization for producing a multitude of electrons in a main or major discharge space prior to the start of a main or major discharge. In this case, to increase the output power of the discharge excitation gas laser apparatus, it is an effective measure to increase a cross-sectional area of a major discharge space. Thus, there have hitherto been proposed gas laser apparatuses having special preionization means for achieving a uniform discharge in a space having a large cross-sectional area. A typical example of this kind of discharge excitation gas laser apparatus is disclosed, for example, in Japanese Patent Laid-Open No. 2-268476. FIG. 13 illustrates an electrodes structure of this apparatus in which a first primary electrode 301 is disposed in a spaced parallel relation with a second primary electrode 302 which is formed of a flat metal plate or board having a plurality of throughholes formed therein. A secondary or auxiliary electrode 303 is disposed at the backside of the second primary electrode 302 with a dielectric member 304 inserted between or sandwiched by them. A plurality of pairs of first and second pin electrodes 305, 306 are disposed at and outside the opposite ends of the first primary electrode 301. Each of the second pin electrodes 306 having an L-shaped cross section is disposed outside and in a face-to-face relation with a corresponding first pin electrode 305 with a limited amount of sparking gap formed therebetween.
The operation of the above-mentioned conventional electrodes arrangement will now be described below. First, let us assume that in FIG. 13, the direction perpendicular to the surface of this figure is the direction of an axis of light and that a resonator for generating a laser beam is provided which comprises a partial reflection mirror and a total reflection mirror disposed before and after the first and second primary electrodes 301, 302 in the direction of the axis of light. Though not illustrated, the first and second primary electrodes 301 302 are connected to an exciting circuit so that when a high voltage is applied across the first and second primary electrodes 301, 302 by means of the exciting circuit, a discharge is caused therebetween to excite a laser gas lying therebetween.
In this regard, a referred to above, it is significant for a discharge excitation gas laser apparatus, which normally operates at a pressure equal to or greater than the atmospheric pressure, to preionize a laser gas used therein. A major feature of this type of gas laser apparatus is that the apparatus includes a first preionization means using a UV spark type preionization system disposed at one side of the first primary electrode 301, and a second preionization means using a corona type preionization system disposed at the other side of the first primary electrode 301. The first preionization means functions to generate a high voltage between the first and second pin electrodes 305, 306 to cause arc discharges whereby ultra violet rays are generated to preionize the laser gas between the first and second primary electrodes 301, 302. The second preionization means functions to generate a high voltage between the secondary electrode 303 and the second primary electrode 302 to cause corona discharges in the respective throughholes in the second primary electrode 302 whereby ultra violet rays are generated to preionize the laser gas between the first and second primary electrodes 301, 302.
In order to efficiently excite the laser gas through discharging, it is necessary to generate a uniform glow discharge, as previously described. In this regard, there is a close relation between the uniform discharge, the density of the preionized electrons and the uniformity in the distribution of density of preionized electrons. Thus, with the UV spark type preionization system, the laser gas is preionized by use of strong ultra violet rays generated by arc discharges, so that a high density of preionized electrons is obtained. In this case, however, it the distances between the paired pin electrodes, which are disposed in a spaced face-to-face relation on the opposite ends of the first primary electrode 301 and act as light sources, are too great, a steep slope will develop in the distribution of preionized electrons density due to the distance dependency of ultra violet rays, so uniform discharge will no longer be generated. In general, with this type of preionization system, since the paired pin electrodes acting as sources for generating ultra violet rays are disposed on the opposite sides of the primary electrodes, the width of length of a main or major discharge is limited for the reasons as referred to above. On the other hand, with the corona type preionization system, preionization is generally performed from the backside of the second primary electrode 302, so uniform distribution of preionized electron density is obtained with respect to the direction perpendicular to the electric field applied for causing a main or major discharge. As a result, the width or length of the primary discharge is not limited. In order to provide a high density of preionized electrons by use of the corona preionization system, it is necessary to increase the density of current for charging a corona preionization capacitor which is formed by the second primary electrode 302, the secondary electrode 303 and the dielectric member 304. The density of current charging the corona preionization capacitor is proportional to the electrostatic capacity of the capacitor as well as the rising speed (i.e., dV/dt) of the voltage applied thereacross. Assuming that the areas of the electrodes are constant for the purpose of increasing the current density, increasing the electrostatic capacity requires that the dielectric member 304 is formed of a material having a high dielectric constant or that the distance between the second primary electrode 302 and the secondary electrode 303 is short. However, the dielectric materials usable for the preionization dielectric member 304 are limited due to a high dielectric break-down voltage which they should have, so the distance between the electrodes 302, 303 can not be reduced to such an extent as required. Thus, it is also difficult to increase the electrostatic capacity of the preionization capacitor. In addition, the rising rate or speed of the voltage applied between the electrodes 302, 303 is limited by the inductance determined by the arrangement or construction of the electrodes, so it is difficult to increase the density of charging current supplied to the preionization capacitor to such an extent as required for providing a high density of preionized electrons by use of the corona type preionization systems. Accordingly, with the above-described conventional electrodes construction having both of the UV discharge type and corona type preionization systems, it is substantially impossible to provide a uniform and high preionization density over an area having a relatively large cross section in a relatively simple manner.
Moreover, since the discharge excitation type gas laser apparatus as described above is of the pulse oscillation type, it is essential to perform repeated operations at such a high rate as from tens to hundreds of shots per second from the point of view of industrial utilization. To enable such repeated operations in a stable manner, impurities such as charged particles produced by main or major discharges have to be removed from the discharging space or vicinity. To this end, the laser gas is generally circulated to facilitate replacement of the used laser gas in the discharging region from one side of the primary electrodes for every discharge. In this case, however, if major discharges are repeated at a rate of more than hundreds of shots per second, circulation of the laser gas has to be performed at a high speed. Accordingly, if there are obstacles such as pin electrodes disposed at one side or both sides of the primary electrodes, vertical flows will develop, thus disturbing the circulating flow of the laser gas. Further, since a lot of impurities are produced by arc discharges and flow into the discharging vicinity, it is impossible to effect sufficient laser gas replacement. As a result, uniform major discharge can not be obtained, reducing the life span of the laser gas.
With the above-mentioned conventional gas laser apparatus in which the preionization sources are disposed on the opposite sides of the first and second primary electrodes 301, 302, the overall construction of the electrodes becomes complicated and hence disadvantageous in lowering the inductance of the main discharge circuit, which is essential for efficient excitation, thus requiring considerable labor in assembly and maintenance of the apparatus.
FIG. 14 shows another conventional discharge excitation type gas laser apparatus, disclosed, for example, in Japanese Patent Laid-Open No. 2-222183. In FIG. 14, the apparatus illustrated includes, in addition to an arc discharge means in the form of paired pin electrodes 405, 406, a corona discharge means in the form of a second primary electrode 402, a secondary or auxiliary electrode 403 and a dielectric member 404, all of which are the same as the corresponding elements 302 through 306 in FIG. 13. It also includes a shunt reactor 407 for equally distributing flows of current among a plurality of pin electrodes 405, 406, a peaking capacitor 408 and a pulsed power source 409. In this case, however, the pin electrodes 405, 406 are disposed, instead of at the opposite sides of the first primary electrode as in FIG. 13, at the backside of the second primary electrode 402. Therefore, the pin electrodes 405, 406 have no adverse effects on laser gas circulation. In this regard, it is to be noted that the pin electrodes 405, 406 are not for preionization but for speeding up the rising of a voltage generated upon corona discharge.
Specifically, as described before, increasing the charging current supplied to a corona preionization capacitor comprising the second primary electrode 402, the secondary electrode 403 and the dielectric member 404 provides an increased density of preionized electrons. In the apparatus of FIG. 14, the preionization capacitor and the peaking capacitor 408 are connected in series to each other. In general, the capacity of the preionization capacitor is much less than that of the peaking capacitor 408, so almost all of the voltage generated by the pulsed power source 409 is applied to the preionization capacitor. For this reason, as compared with usual parallel connection of the preionization capacitor and the peaking capacitor 408, the serial connection of these capacitors provides a very fast rising of a voltage applied to the preionization capacitor and hence an accordingly increased charging current. In this manner, the pin electrodes 405, 406 of the conventional apparatus of FIG. 14 generates a voltage of a fast rising rate between the second primary electrode 402 and the secondary or auxiliary electrode 403 but does not perform preionization.
In summary, the above-described conventional discharge excitation gas laser apparatuses employ a plurality of preionization systems for preionizing a laser gas in a discharge excitation space so as to achieve a large area discharge. In these gas laser apparatuses, a plurality of preionization means are disposed at such locations as interrupt portions of the circulating flows of the laser gas, as illustrated in FIG. 13, endering it difficult to obtain uniform discharge in a stable manner during repeated operations of the apparatus, as would be the case in industrial applications. Additionally, in either examples as illustrated in FIGS. 13 and 14, preionization means are disposed at locations adjacent the opposite sides of the first and second primary electrodes, thus rendering the overall electrodes construction relatively complicated. This not only prevents lowering of the inductance of the discharge loop or circuit, which is needed for highly efficient laser oscillation, but also requires considerable labor for maintenance.
Further, FIG. 15 illustrates a conventional excitation circuit in a discharge excitation pulsed laser apparatus as disclosed, for example, in a reference entitled "Excimer Laser Developments and Application Techniques" by Shuntaro Watanabe, Page 17. This excitation circuit is generally called a capacity transfer type excitation circuit which includes a pair of first and second primary electrodes 501, 502 disposed in a spaced parallel relation, a power supply terminal 503 for supplying electric power to the excitation circuit, a storage capacitor 504, a plurality of peaking capacitors 505, a switch 506, a charging coil 507, and a plurality of pairs of preionization pin electrodes 508 disposed on the opposite sides of the electrodes 501, 502.
In operation, a high voltage is first imposed on the power supply terminal 503 to charge the storage capacitor 504 through the charging coil 507. When the switch 506 is turned on, a voltage equal to the charged voltage of the storage capacitor 504 is imposed across the charging coil 507 and between the preionization pin electrodes 508. Since the charged voltage of the storage capacitor 504 is sufficiently higher than a dielectric break-down voltage between the opposed preionization pin electrodes 508, arc discharges are caused between the opposed pin electrodes 508, generating ultra violet rays which preionize a laser gas in a major discharge space defined between the first and second primary electrodes 501, 502. In this manner, ionized electron seeds are uniformly generated in the major discharge space. As arc discharge begins to develop, a gap between each pair of opposed pin electrodes 508 becomes conductive, forming a closed circuit comprising the storage capacitor 504, the peaking capacitor 505, the switch 506 and the opposed pin electrodes 508. The inductance of the closed circuit is set to be far less than the inductance of the charging coil 507, so charged electrons stored in the storage capacitor 504 transfer to the peaking capacitor 505, rapidly charging it. When the charged voltage across the peaking capacitor 505, which is equal to the voltage applied between the first and second primary electrodes 501, 502, reaches the dielectric break-down voltage across the primary electrodes 501, 502, major discharge initiates from the charged electron seeds. Upon initiation of the major discharge, charged electrons stored in the peaking capacitor 505 begin to rapidly rush into the major discharge space between the primary electrodes 501, 502 to thereby excite the laser gas therein.
The apparatus of FIG. 15 is called an automatic preionization type in which a preionization circuit for applying a high voltage between the opposed pin electrodes 508 is incorporated in the excitation circuit so that preionization and major discharge are automatically carried out in suitable timing upon energization of the excitation circuit. The automatic preionization system is simple in construction and inexpensive to manufacture, so it has become popular and been widely employed with a laser apparatus for industrial applications.
FIG. 16 shows a further conventional discharge excitation gas laser apparatus utilizing X rays for preionization, as disclosed, for example, in a reference entitled "X-Ray Preionization of Rare-Gas-Halide Lasers", by K. Midorikawa et al, IEEE J. Quantum Electron. QE-20, NO 3, P198, 1984. In this figure, a pair of first and second primary electrodes 601, 602 are disposed in a spaced parallel relation with respect to each other, as in the apparatuses of FIGS. 13 and 14. A pulse forming line 609, which is charged by a pulsed charger 610, has an impedance matching that of a major discharge space, which is defined between the first and second primary electrodes 601, 602, for storing energy to be supplied to the major discharge space. A switch in the form of a railgap 611 switches the connection between the pulse shaping passage 609 and the first primary electrode 601 on and off. The apparatus further includes an X-ray generator 612 for generating X rays toward the major discharge space or gap between the first and second primary electrodes 601, 602. It also includes a Marx generator 613 for applying a high voltage to the X-ray generator 612, a trigger generator 614 for generating a trigger signal to the Marx generator 613 and the pulsed charger 610 to trigger them, and a delay circuit 615 connected between the trigger generator 614 and the pulsed charger 610 for supplying the trigger signal from the trigger generator 614 to the pulsed charger 610 at delayed timing. The timing of preionization and the timing of major discharge are determined by the trigger generator 614 and the delay circuit 615.
The operation of the above apparatus will be described below. First, the trigger generator 614 concurrently generates two reference signals, one of which is fed to the Marx generator 613 whereupon it generates a high voltage to the X-ray generator 612. The X-ray generator 612 generates X rays which are radiated on the laser gas between the first and second primary electrodes 601, 602 to preionize the same. On the other hand, the other of the two reference signals is fed to the delay circuit 615 and thence to the pulse charger 610 with a prescribed time delay which is set by the delay circuit 615, whereby the pulse forming line 609 is charged by the pulsed charger 610. When the charged voltage of the pulse forming line 609 exceeds a dielectric break-down voltage between opposed pin electrodes in the railgap 611, a plurality of arc discharges take place between the railgap electrodes, thereby performing switching operation. Simultaneous with the start of the switching operation of the railgap 611, the voltage imposed on the first primary electrode 601 begins to increase rapidly. When the voltage applied to the first primary electrode 601 exceeds the dielectric break-down voltage between the first and second primary electrodes 601, 602, a primary or major discharge therebetween starts. At this moment, the energy stored in the pulse forming line 609 is released or discharged into the major discharge space between the first and second primary electrodes 601, 602 to excite the laser gas therein.
FIG. 17 illustrates, in a graphical representation, the preionization timing dependency of the laser output as disclosed in a reference entitled "Parametric Study of X-Ray Preionized High-Pressure Rare Gas Halide Lasers", by M. Steyer and H. Vogas, Appl. Phys. B42, P155-160, 1987. In this graph, the abscissa represents the preionization timing which is given by a length of time elapsed from the start of X-ray radiation until the time when the laser pulse reaches a peak value, and the ordinate represents the laser output. From this graph, it can be seen that the laser output greatly depends on the preionization timing, so there exists an optimal timing for preionization. FIG. 18 graphically illustrates the result of measurements in which the optimal value for the preionization timing is plotted against the charged voltage of the primary capacitor in the excitation circuit. FIG. 19 graphically illustrates the result of measurements in which the optimal value for the preionization timing is plotted against the concentration of krypton in the laser gas composition. From FIGS. 18 and 19, it can be seen that the optimal value for the preionization varies in accordance with the operating conditions of the laser apparatus such as the composition of the laser gas, the method of applying a voltage between the first and second primary electrodes 601, 602 and the like.
In the preionization system as illustrated in FIG. 15, the preionization timing is automatically determined by a circuit constant such as the circuit inductance, which is determined by the capacitance and structure of the capacitor, and the like. Though the optimal timing for preionization varies according to the laser operating condition of the apparatus as referred to above, changing the preionization timing in the preionization system of FIG. 15 requires a change in the circuit constant, so it is quite difficult to always maintain the preionization timing at an optimal timing irrespective of the laser operating condition of the apparatus. On the other hand, with the conventional apparatus of FIG. 16, the circuit for major discharge and the circuit for preionization are formed separately and independently of each other, and the timing for major discharge and the timing for preionization are properly controlled by means of the delay circuit 615, but no independent setting of preionization timing is carried out. That is, the preionization timing dependency of the laser output power is measured to find an optimal preionization timing at which the laser output power becomes the greatest, and a delay time is set to provide the optimal preionization timing.
In order to use a discharge excitation pulse laser apparatus for industrial applications, it is essential for the laser apparatus to continuously operate for an extended period of time, as referred to above. During continued operation of the apparatus, the laser operating condition of the apparatus varies with time because gradually increasing degradation of the laser gas, thus resulting in a gradual change in the optimal timing of preionization. With the above-mentioned conventional apparatus, however, the preionization timing is fixedly set or even if it can be adjusted, it is necessary to monitor the laser output power at all times during operation. In this connection, to always monitor the laser output power, part of a laser beam must be splitted or separated from the remaining major portion thereof by optical means such as a beam splitter, and this reduces the efficiency or the overall output power of the apparatus. In addition, special means such as a laser sensor or meter are required for sensing or measuring the laser output power. This results in a complicated construction of the apparatus and high costs for manufacture, and requires extra labor on the part of the operator.